1. Field of the Invention
This invention relates to an improved catalyst useful in the conversion of aromatic hydrocarbons, particularly for the isomerization of alkylaromatics.
2. General Background
Bound zeolite catalysts are widely used for hydrocarbon conversion reactions, based on their high activity and/or selectivity. Such catalysts are particularly useful in conversions involving aromatic hydrocarbons: synthesis of aromatics from paraffins and naphthenes, alkylation of aromatics with light olefins, transalkylation and isomerization. A major driving force in the development of synthesis, transalkylation and isomerization catalysts has been the growth in demand for para-xylene as an intermediate in polyester manufacture.
C.sub.8 aromatics which have been synthesized and recovered in an aromatics complex contain a mixture of the three xylene isomers and ethylbenzene. Para-xylene normally is recovered in high purity from the C.sub.8 aromatics, for example by adsorption or crystallization, and other isomers may be separated as well. Para-xylene generally constitutes only 15-25% of the mixture, but accounts for most of the demand for C.sub.8 aromatics. Normally, it is desirable to minimize the amount of feedstock required for a given quantity of para-xylene. Therefore, it is industrially significant to isomerize the para-depleted raffinate from para-xylene recovery to adjust the isomer balance toward thermodynamic equilibrium for recycle to the recovery section.
An increasingly close approach to equilibrium of C.sub.8 aromatic isomers in an isomerization process is associated with higher losses of C.sub.8 aromatics to other hydrocarbons. A close approach to equilibrium minimizes the amount of recycle to para-xylene recovery, and thus reduces the investment and operating costs of the complex. A lower loss of C.sub.8 aromatics reduces feedstock requirements and increases the proportion of higher-value products. The performance of an isomerization catalyst is assessed principally by the balance of C.sub.8 aromatic losses for a given approach to equilibrium and by its stability while achieving such performance.
Numerous catalysts for isomerizing C.sub.8 aromatics have been disclosed, and many of them involve the use of a crystalline aluminosilicate zeolite-containing catalyst. Crystalline aluminosilicates, generally referred to as zeolites, may be represented by the empirical formula: EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
in which n is the valence of M and x is generally equal to or greater than 2.
Zeolites have skeletal structures which are made up of three-dimensional networks of SiO.sub.4 and AlO.sub.4 tetrahedra, corner linked to each other by shared oxygen atoms. Zeolites particularly suited for use as isomerization catalysts include mordenite and the ZSM variety. In addition to the zeolite component, certain metal promoters and inorganic oxide matrices have been included in isomerization catalyst formulations. Examples of inorganic oxides include silica, alumina, and mixtures thereof. Metal promoters, such as those of Group VIII or Group III metals of the Periodic Table, have been used to provide a dehydrogenation functionality. The acidic function can be supplied by the inorganic oxide matrix, the zeolite, or both.
Catalysts for isomerization of C.sub.8 aromatics ordinarily are characterized by the manner of processing ethylbenzene associated with the xylene isomers. Ethylbenzene is not easily isomerized to xylenes, but it normally is converted in the isomerization unit because separation from the xylenes by superfractionation or adsorption is very expensive. In older isomerization technology, ethylbenzene was transalkylated with resulting product loss to heavy aromatics.
One modern approach is to react the ethylbenzene to form a xylene mixture in the presence of a solid acid catalyst with a hydrogenation-dehydrogenation function. An alternative approach is to dealkylate ethylbenzene to form principally benzene while isomerizing xylenes to a near-equilibrium mixture. The former approach enhances xylene yield by forming xylenes from ethylbenzene, but the latter approach commonly results in higher ethylbenzene conversion and thus lowers the quantity of recycle to the para-xylene recovery unit. The latter approach also yields a high-quality benzene product. The effectiveness of a catalyst applied in the latter approach thus is measured by:
activity, in approaching an equilibrium mixture of xylene isomers, and achieving ethylbenzene conversion, and PA1 selectivity in both xylene isomerization and ethylbenzene conversion, minimizing side reactions such as transalkylation, disproportionation, demethylation, and hydrogenation and subsequent cracking of the aromatic ring.
In particular, the ethylbenzene conversion should yield benzene and ethane rather than transalkylating the ethyl group to another aromatic ring. The catalyst of the present invention is applied to advantage in this approach.